The Section tool doesn't work if you want external geo from something in front of the clip. And currently we can't select external geo thru a solid.why don't you just click on the Sketcher_ViewSection tool/icon in sketcher wb.
It would be useful to be able to select external geo under the mouse in a solid, FreeCAD can see the external shapes we just can't pick one. It's possible if the picks were filtered to vertex and lines, that might do it, not sure.
Here is a pic where I wanted the external geo from the front single hole to the rear sketch. Because I rotated in 3D I can see the sketch clear to do external geo but I can't select it thru the solid. I feel that's the reason SM1 is being used as a default.
It's possible that being able to switch back and forth SM1-SM2 realtime, is the best case.